Four-Stroke vs. Six-Stroke Engine: Key Differences

A heat engine converts thermal energy into mechanical energy. There are various types of heat engines based on different criteria. Based on working cycles, heat engines are categorized into 2-stroke, 4-stroke, and 6-stroke engines. Four-stroke and six-stroke engines are types of internal combustion (I.C.) engines used to convert fuel into mechanical energy. They differ in the number of strokes or phases in their operating cycles, which are the sequences of events that occur within the engine to produce power. Here’s an overview of both types.

Four-Stroke Engine

In a four-stroke engine, four events take place inside the engine cylinder: suction, compression, power, and exhaust. All these events are completed in four strokes of the piston or in two revolutions of the crankshaft. Hence, it is known as a four-stroke engine. The four cycles are:

• Intake Stroke: The engine takes in a mixture of air and fuel through the intake valve as the piston moves downward.
• Compression Stroke: The piston moves upward, compressing the air-fuel mixture, which increases its pressure and temperature.
• Power Stroke: A spark plug ignites the compressed air-fuel mixture, causing an explosion that forces the piston down. This is the stroke where power is produced.
• Exhaust Stroke: The piston moves back up while the exhaust valve opens, allowing the burned gases to be expelled from the cylinder.

In a two-stroke engine, the whole sequence of events (i.e., suction, compression, power, and exhaust) are completed in two strokes of the piston, i.e., one revolution of the crankshaft. It’s the most common type of internal combustion engine and is widely used in cars, motorcycles, and many other applications.

Six-Stroke Engine

The six-stroke engine is developed by modifying a four-stroke design to improve efficiency and reduce emissions. It is a type of internal combustion engine based on the four-stroke engine with additional complexity. There are two power strokes: one with fuel and the other with steam or air.

The six-stroke engines are categorized into single-piston type and opposed-piston type. The single-piston type uses one piston in the cylinder. The opposed-piston design uses two pistons per cylinder operating at different rates, with detonation occurring between the pistons.

Single-piston designs include the Griffin six-stroke engine, Bajulaz six-stroke engine, Crower six-stroke engine, and Velozeta six-stroke engine, etc. Opposed-piston designs include Beare head, M4+2, etc. Other two-piston designs are also available.

The following are the six stages in a six-stroke engine having four valves: a suction valve, an air inlet valve, an exhaust valve, and an air exhaust valve.

• Suction
• Compression
• Ignition
• Exhaust
• Air Suction
• Air Exhaust

The first four strokes are the same as either the Otto or diesel cycle. Energy is produced in the power stroke. After the end of the four strokes, the bore will still be hot enough to expand water. Hence, after the end of the fourth stroke, when the piston is moving towards BDC (Bottom Dead Center), water is inserted inside the cylinder, and the water is expanded by the addition of heat. It is considered to be the fifth stroke. This stroke also produces power. Hence, in a cycle, power is produced twice.

After the fifth stroke, the piston is reciprocated back towards TDC (Top Dead Center), which is considered to be the sixth stroke. Water is injected at the end of the exhaust, so that an extra power and exhaust stroke is added.

The six-stroke engine is still largely experimental and hasn’t seen widespread commercial use. It aims to improve efficiency by extracting more energy from the same amount of fuel, potentially reducing emissions and fuel consumption. However, it’s more complex and not as commonly used as the four-stroke engine.

Difference Between Four-Stroke and Six-Stroke Engines

The following table summarizes the differences between four-stroke and six-stroke engine types with respect to various features:

Conclusion

Four-stroke engines have four phases (intake, compression, power, exhaust) per cycle, with one power stroke, are widely used in various applications, and are known for their simplicity but may have lower efficiency. In contrast, six-stroke engines feature an experimental design with six phases (intake, compression, power x2, exhaust x2) per cycle, offering the potential for higher efficiency due to two power strokes, lower emissions, and reduced fuel consumption, but they are more complex and not widely used in practical applications as they are still in the experimental stage.